This invention relates to a device for providing improved backlighting for liquid crystal displays (LCDs).
With the proliferation of mobile and handheld computing devices, the need for these devices to work under a wider range of environmental conditions increases. These devices typically use liquid crystal displays because of they are inexpensive to manufacture in high quantities and high qualities. One particular problem with LCDs is that they can be difficult or impossible to read without additional an light source. For this reason, most LCDs utilize a lamp that is mounted to a circuit board behind the display (hence xe2x80x9cbacklightingxe2x80x9d), although lamps may be mounted in front of the display.
Backlighting may be provided by several different methods including the use of electroluminiscent lamps (ELs), light emitting diodes (LED) and cold cathode fluorescent lamps (CCFL). CCFLs are a popular choice for several reasons including the fact that CCFLs do not produce a large amount of heat for the amount of uniform, bright white light they produce. LEDs are not sufficiently efficient in their light output. CCFLs have a lifetime in the neighborhood of 20,000 to 25,000 hours and unlike ELs, are not sensitive to high humidity. For these reason, CCFLs are the preferred choice for backlighting mobile and handheld LCDs.
One of the drawbacks of CCFLs is that the output or luminance of the lamp drops dramatically as the operating temperature shifts out of the normal operating temperature of the CCFL. For example, below about 0xc2x0 C. the CCFL would not be able produce enough light to make the LCD readable. In general, a backlight needs to put out roughly 4000 to 4500 candela/m2 (cd/m2) in order for the LCD to be readable. Because LCDs typically attenuate all but 7% of the light put out by the backlight, this out put by the backlight translates into a luminance of the LCD of about 280 to 315 cd/m2.
This is a particular problem for devices that are usually outdoors in colder climates. For example, global positioning system units, gas pumps and automobiles. This is also a problem for a much wider range of devices used in perpetually cold environments such as above the Arctic and Antarctic circles and in outer space. For example, in a LCD in a automobile on a cold winter morning would appear dim and unreadable until the lamp heated up to it normal operating temperature, i.e., room temperature.
One common solution to this problem is to wrap a fine heating element or wire around the CCFL to help raise the temperature of the lamp when the ambient temperature is cold. While this method is effective, it is nonetheless an unsatisfactory solution because of the complexities of wrapping a fine wire around a small diameter CCFL. Such a process is not amenable to high volume production. Consequently, heated CCFLs have been a luxury item until this point. This has limited their use in consumer electronics and prevented them from becoming standard features on automobiles.
Another drawback of wire wrapped CCFLs is that the wire blocks a significant portion of the light of the lamp. To compensate for this loss of light and hence luminance, additional or more powerful lamps must be used, which adds costs to the backlighting system in the display.
Another solution to the problem of unsatisfactory luminance in cold temperatures is the use of self-heating lamps. These lamps utilize gases under high pressure to increase the resistance of the gas, which in turn produces more heat. However, self-heating lamps do not perform well at even moderately elevated ambient temperatures because the temperature of the lamp cannot be easily lowered without turning the lamp off. The elevated temperature causes dimming.
Consequently, the inventors have recognized the need for an improved heated CCFL to overcome one or more of the above discussed problems.
The represent invention relates to a method of backlighting a liquid crystal display which includes heating a fluorescent lamp with an external heating element which does not contact the lamp. The heating step may be accomplished by passing a current through a resistive material. The resistive material may be coated on to a surface. The present invention also relates to a backlighting system with a fluorescent lamp and an external heating element which does not contact the fluorescent lamp. The heating element may be a resistive material coated on a substrate. The present invention also relates to a portable display device which includes a liquid crystal display, a fluorescent lamp to act as a backlight, a heating element which does not contact the fluorescent lamp and optionally including reflectors, diffusers, and optical enhancement films. The heating element may be a resistive material coated on a substrate.